124 G. E. COGHILL 



passes out of the tract from caudad as an axone or arises as a 

 collateral from the ascending process of a tract cell. 



Traced caudad in this embryo the fibers of the motor column 

 can be identified as far as the fourteenth myotome. Beyond this 

 a single fiber may be seen in occasional sections as far as the 

 seventeenth myotome. Ventral root fibers can be recognized as 

 far caudad as the thirteenth myotome, that is, there are at least 

 twelve pairs of ventral roots in this specimen. 



The root fiber indicated in figure 2 ( VF) goes to the fifth myo- 

 tome of an embryo of the same physiological age as that from 

 which figure 1 was taken. Although the cell body of the neurone 

 from which the root fiber arises does not appear in this section, 

 the tract fiber has the appearance of a descending process (DP). 

 The slight outward bend of the fiber at the point of origin of the 

 collateral which passes into the nerve root is characteristic of 

 this early stage of development. 



Slightly rostrad of the root fiber in this section is a character- 

 istic nucleus of the motor column and its perikaryon can be 

 Indistinctly seen (VC). The peripheral end of this cell spreads 

 out against the external limiting membrane and extends rostrad 

 and caudad in ascending and descending processes. 



Root fibers occur in this embryo as far caudad as the eighth 

 myotome, and uncertain suggestions of roots appear at the levels 

 of the ninth and tenth myotomes. The fibers of the latero-ventral 

 tract may be traced caudad as far as the twelfth myotome. 



2. Embryos of the early flexure stage 



Conditions found in embryos very soon after they first respond 

 to tactile stimulation are represented in figures 3 to 9, inclusive. 



The section from which figure 3 is taken is in an approximately 

 frontal plane, tipped slightly ventro-caudad and slightly dorso- 

 laterad on the side figured, so as to leave a point of the latero- 

 ventral portion of the spinal cord between the notochord and 

 the eighth myotome. The descending process of a neurone (DP), 

 the nucleus of which is not included in this section, branches into 

 two divisions close to the external limiting membrane. One 



